How to calculate timedelta in dataframe column

Question

I want to add a new column in a dataframe that show a new date in column which is date in b column add months in a column.

a b
3 2021-08-01
2 2021-07-01
5 2021-01-01

a b c
3 2021-08-01 2021-11-01
2 2021-07-01 2021-09-01
5 2021-01-01 2021-06-01

I have try

pd.to_datetime(df['b'] + timedelta(df ['a'])

But it doesn't work.

Answer 1

Let us do

df['new'] = df.apply(lambda x: x['b'] + pd.offsets.MonthBegin(x['a']), axis=1)
Out[194]: 
0   2021-11-01
1   2021-09-01
2   2021-06-01
dtype: datetime64[ns]